On returning to Canada, he became a senior civil servant in Saskatchewan before entering politics in 1960 and serving as a cabinet minister in the governments of Tommy Douglas and Woodrow S. Lloyd.[1] As minister of health, he played a crucial role in the introduction of Medicare.[2][3]

In 1970, Blakeney succeeded Lloyd as leader of the Saskatchewan NDP, which was then in opposition. Historically, his election as leader has been interpreted as a victory of the provincial NDP's "establishment" over its extreme left-wing Waffle faction, because Lloyd was trying to move the party more to the political left, which meant he supported the Waffle Manifesto at the October 1969 Federal NDP convention in Winnipeg.[4] As well, Lloyd backed The Waffle's right to debate issues after the convention which disturbed many of his MLAs, who eventually forced him to resign in March 1970.[4]

Blakeney's government practised state-led economic intervention in the economy.

The farmers were a high priority, as globalization began transforming agriculture, weakening the traditional family farm through consolidation, mechanization, and corporatization. The NDP promised a "revitalized rural Saskatchewan," and Blakeney's introduced programs to stabilize crop prices, retain transportation links, and modernize rural life. Looking back he lamented the gradual conversion of Saskatchewan's family farms into larger agricultural ventures: Without resorting to the "very high costs" and "billions of dollars" used to preserve family farming in Europe and the United States, "[w]e were, it seems, King Canute trying to hold back the tide."[5]

His government created a Crown corporation in the potash industry in an attempt to further diversify the province's agrarian economy and threatened expropriation of private potash mines within the province.[1] Blakeney pointed out that the sums paid for these mines were slightly in excess of their appraised "book" value. However, the mere threat of expropriation created a political firestorm that involved even the U.S. government.[6]

Blakeney also created a state-owned oil and gas corporation, SaskOil, to handle oil exploration and production.[1] The private oil industry had essentially abandoned Saskatchewan following the NDP's imposition of extremely high royalty rate policy of the early 1970s. Prime Minister Trudeau's policies (to centralize control in Ottawa) outraged Blakeney, and he moved closer to Alberta's position of open hostility. Blakeney joined Alberta Progressive Conservative Premier Peter Lougheed in a fight for provincial rights over minerals, oil and gas.[1][7]

Blakeney played an important role in the federal-provincial negotiations that led to the 1982 patriation of the Canadian constitution.[1]

Once in opposition, Blakeney continued to lead the party up to the 1986 provincial election. The NDP not only regained much of what it had lost in its severe beating of four years earlier, but also gained more votes overall than Devine's Progressive Conservatives.[1] However, much of that margin was wasted on landslide margins in Regina and Saskatoon, leaving the NDP eight seats short of regaining power. Blakeney resigned in 1987 to be succeeded by Roy Romanow.

As a private citizen Blakeney served as a consultant to the Romanow government in the 1990s when they sold the SaskOil to Occidental Petroleum. Then Blakeney served on the board of directors of the successor corporation.